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Tesi etd-03212017-103316


Thesis type
Tesi di dottorato di ricerca
Author
SICONOLFI, LORENZO
URN
etd-03212017-103316
Title
Stability and sensitivity analysis for flow control
Settore scientifico disciplinare
ING-IND/06
Corso di studi
INGEGNERIA INDUSTRIALE
Commissione
tutor Prof. Camarri, Simone
correlatore Prof.ssa Salvetti, Maria Vittoria
Parole chiave
  • Stability analysis
  • complex flows
  • sensitivity analysis
  • noise amplifiers
  • oscillators
Data inizio appello
20/04/2017;
Consultabilità
parziale
Data di rilascio
20/04/2020
Riassunto analitico
Wakes, jets and boundary layers are examples of open flows, where fluid particles are<br>convected downstream outside the physical domain of interest. These flows can exhibit<br>several types of instabilities, depending on the considered geometry and flow conditions. A<br>common classification in open flows is made according to the features of their instabilities.<br>Flows are called noise amplifiers, or amplifiers, when the instabilities are the result<br>of strong amplifications of external disturbances, that also define their characteristics.<br>Conversely, oscillators are flows that show an intrinsic dynamics and, under specific<br>conditions, synchronised self-sustained oscillations occur. The characterization of the<br>instability mechanisms is the fundamental stage to design effective and efficient control<br>strategies.<br>In this thesis, the study of amplifiers and oscillators is carried out in the framework of<br>the linear stability analysis. Concerning the amplifiers, the study is here addressed to<br>investigate passive methods for transition delay in a Blasius boundary layer, aimed at<br>a reduction of the friction drag. All the control strategies here considered are based<br>on the methods of the spanwise mean velocity gradient, where the laminar-turbulent<br>transition is delayed through a modulation of the velocity inside the boundary layer in<br>spanwise direction. Different control devices are investigated in detail through the use<br>of direct numerical simulations and local stability analyses. The results of the stability<br>problem are summarized in neutral stability curves, that allow to identify promising<br>configurations in terms of stabilization of the Blasius unstable mode.<br>The study of oscillators is here conducted characterizing the leading global unstable modes<br>that define the flow behaviours. The flow in micromixers is first of all investigated using<br>global stability analysis. In particular, starting from the well documented T-junction<br>configuration, variations in geometry and fluid properties are here considered to assess<br>their effects on the onset of different flow regimes. Moreover, the global stability approach<br>is also applied to the flow past a sphere, in order to characterize its second bifurcation<br>that drives the system from a steady asymmetric solution towards an unsteady flow<br>state. Finally, a theoretical work is presented, in which an accurate estimation of the<br>global stability modes and of the characteristics of wakes, under assumption of slowly<br>non-parallel flow, is obtained by a higher-order correction term in the WKBJ asymptotic<br>approximation.
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